This invention relates to improvements in tractor-trailer fifth wheel couplings and, more particularly, to apparatus for protecting a low-friction, plastic bearing plate cover from damage during coupling between a tractor and trailer.
To obviate the need for repeatedly lubricating a fifth wheel tractor-trailer coupling, it is known to provide the tractor's fifth wheel bearing plate with a low friction plastic covering, e.g. coverings of certain ultra high molecular weight polyetheylenes and polypropylenes. See, for example, the disclosures of U.S. Pat. Nos. 4,169,635 and 3,704,924. Such coverings are particularly preferred due to their exceptionally low coefficient of friction and durability. These plastic coverings are commonly held in place on the fifth wheel bearing surface by screws, rivots and/or adhesives.
During the coupling between tractor and trailer, the mating bearing surfaces often impact each other suddenly and with tremendous force. Such impact can cause tearing or shearing of a plastic covering on one or the other of such mating surfaces. To protect these plastic coverings, it has been proposed to provide two protective "riser bars" on the outside peripheral edges of the tractor's fifth wheel bearing plate and its associated sloped flanges. See, for example, the disclosure of U.S. Pat. No. 4,542,912. Such riser bars are substantially flush with the surface of the plastic covering and extend from the forward edge of the sloped flanges backwards along the circular portion of the bearing plate to a point at which the bearing plate is pivotally mounted to the tractor body. The riser bars are positioned in cutout portions of the plastic covering to allow replacement of the covering without disturbing the riser bars. According to the author, such peripherally located riser bars are intended to both guide and pivot the fifth wheel attachment of the tractor so that it is at the correct alignment and location with respect to the trailer during coupling.
In utilizing fifth wheel assemblies having protective riser bars of the type described above, it has been observed that the riser bars, owing to their location, thickness and shape, eventually wear arcuate grooves in the trailer's mating bearing surface (commonly known as the "bolster" plate). Moreover, it has been observed that, perhaps as a result of the bolster plate wearing unevenly and the fact that the riser bars do not extend rearwards beyond the bearing plate pivot axis, there is a tendency for the bolster plate to gouge and tear the protective plastic covering on the tractor's fifth wheel bearing plate. Being located along the outer edge of the substantially circular portion of the tractor's bearing plate, and being flush with the upper surface of the bearing plate's plastic covering, the riser bars frequently contact the trailer's bolster plate at times other then tractor/trailer coupling. For example, normal rocking of the trailer about a horizontal axis extending parallel to the trailer length causes the riser bars to repeatedly impact against the bolster plate. Also, as the tractor turns relative to the trailer to follow curves in the road, the risers bars rub against, and thereby scrape, the trailer's bolster plate. This action is believed to give rise to the observed arcuate grooves in the bolster plate. Further, since the riser bars do not extend beyond the aforementioned pivot axis, they afford no protection from the trailer's bolster plate after the tractor's bearing plate tips (pivotal) to a horizontal seating position during tractor/trailer coupling.